Gemara

R. Musiya: If a person tells something to another, it is forbidden for the latter to tell anyone else without permission - "Hash-m spoke to Moshe from the Ohel Mo'ed Leimor".

Rashi: Lo Emor (do not say) the matter without permission.

Rashash: Pesachim 42a expounds the word "Leimor" to mean "Lo Ne'emar" or "Lav Amor" (i.e. there is a Lav).

Sanhedrin 29a (Mishnah) Question: What is the source that afterwards, a judge may not tell Ploni 'I wanted to acquit you, but they outnumbered me'?

Answer: It says "Lo Selech Rachil b'Amecha" and "Holech Rachil Megaleh Sod".

R. Ami expelled a Talmid from the Beis Medrash for repeating something that was said there 22 years earlier, and announced 'This person reveals secrets.'

Erchin 16a - Rabah bar bar Chanah: Anything said in front of three people is not Lashon ha'Ra (one may tell it to others).

Rishonim

Rif (Sanhedrin 9a) and Rambam (Sanhedrin 22:7) and Rosh (Sanhedrin 3:33) bring the Mishnah in Sanhedrin.

Rif (Sanhedrin 9a) and Rambam (Sanhedrin 22:7) and Rosh (Sanhedrin 3:36): A Talmid was expelled from the Beis Medrash for repeating something that was said there 22 years earlier. They announced about him 'This person reveals secrets.'

Question: Perhaps R. Ami did so because the Talmid intended to spread the word!

Answer (Maharshal on SMaG Sof Lav 9): The Gemara says 'he revealed'. This does not connote intent to spread the matter.

She'altos (Va'Yeshev 28, b'Sof): Normally, one needs permission to repeat anything he was told. Rabah bar bar Chanah teaches that if it was told in front of three, this is like permission.

Note: Ha'Emek She'alah says that Rashi (Erchin Reish 16a) explains similarly. The Rashbam (Bava Basra Sof 39a) and Tosfos (Erchin 15b DH Kol) explain Rabah's teaching differently.

Hagahos Maimoniyos (De'os 7:7, cited by Magen Avraham OC 156:2): It is Lashon ha'Ra to repeat (without permission) what one was told.

Hagahos Maimoniyos (De'os 7:8, also cited by Magen Avraham): If the matter was told to three people they may repeat it unless they were told not to.

Source: We learn this from the episode in Sanhedrin. Presumably, the matter was said in front of three, but they were told not to repeat it.

Poskim

Shulchan Aruch (CM 19:1): After the deliberation, they call in the parties and the greatest judge tells them the verdict. Afterwards, a judge may not tell Ploni 'I wanted to acquit you, but they outnumbered me'. It says about one who does so "Holech Rachil Megaleh Sod".

Beis Yosef (DH u'Mah she'Chosav): The parties were not present during the deliberation in order that they will not know who was Mezakeh and who was Mechayev.

SMA (1): The greatest judge tells them the verdict lest they suspect that the one who said the verdict did so because he agrees with it, e.g. that Ploni is liable.

Question: Obviously, Hash-m would not be harmed if Moshe repeated what he was told. Why then was it prohibited for Moshe to say it over, had Hash-m not specifically told him to do so?

Answers (Be'er Mayim Chayim 2:27): It is proper to accustom oneself not to repeat things even if there will be no harm to the teller, lest one repeat them when there would be harm.

Alternatively, the Isur to repeat things when there will be no harm is only if it was said privately in the teller's house (similar to Hash-m; His voice was not heard outside the Ohel Mo'ed). When someone tells something over privately, it suggests that the teller does not want it repeated.

Chafetz Chaim (2:7): If a matter was told to three people and they were told not to repeat it, even if two repeated it, 'Bal Yomar' still applies to the third.

The Chafetz Chaim is unsure whether or not the same applies if it was told to four and three repeated it.